1. Field of the Invention
The present invention relates to a method for conveying silicon fragments. The present invention also relates to a vibrating conveyor.
2. The Prior Art
High-purity silicon is produced, for example, by chemical vapor phase deposition of a high-purity chlorosilane gas on a heated substrate. The silicon is deposited in the form of polycrystalline rods. A large proportion of the monocrystalline silicon which is required for the semiconductor industry is produced from these silicon rods using the Czochralski crucible-pulling process.
In this process, a melting crucible is filled with broken pieces of silicon, referred to below as silicon fragments, with typical diameters of between 5 and 150 mm. The silicon fragments are melted, and a mono-crystalline silicon bar is pulled out of the molten material by means of a monocrystalline seed. The silicon fragments which are needed to fill the melting crucible are obtained by breaking up the polycrystalline bars produced during the vapor phase deposition. The breaking tools used are, for example, hammers, chisels or jaw or rolling crushers made from metal.
After the hard, brittle silicon rods have been broken up, it is possible to detect contaminating particles and foreign atoms on the sharp-edged surfaces of the fragments. The particles are generally dust which is produced during crushing of the silicon and during conveying of the fragments. The foreign atoms result in particular from the crushing tools.
This contamination must be removed before the silicon fragments are melted in the melting crucible. This is preferably carried out by means of a material removing etching treatment using acids or acid mixtures, followed by rinsing with water.
The large specific surfaces of the sharp edged fragments have serious drawbacks, resulting in particular in
a high consumption of acid for material-removing treatments,
a high level of acid entrainment due to the film of acid adhering to the surface when the silicon fragments are transferred to a rinsing bath, with associated contamination of the rinsing water, and
etch-back of the broken edges.
It is therefore, an object of the present invention to provide a method and a device which reduce the abovementioned drawbacks of sharp edged silicon fragments.
The above object is achieved according to the invention by means of a method for conveying silicon fragments wherein the silicon fragments are guided over a conveyor surface, which is made from hyperpure silicon, of a vibrating conveyor.
The above object is also achieved according to the invention by means of a vibrating conveyor, wherein the conveyor surface is made from hyperpure silicon.
Surprisingly, it has been found that the sharp edged silicon fragments become rounded when they are conveyed on the vibrating conveyor surface of a vibrating conveyor. The fact that the specific surfaces of the silicon fragments are reduced and contamination adhering to the surface is ground off has proven particularly advantageous in the present invention. According to the invention, the silicon fragments are conveyed on a conveyor surface which is made from hyperpure silicon and is preferably of planar, trough-like or tubular construction. In the context of the present invention, hyperpure silicon is understood to mean monocrystalline or polycrystalline silicon with a purity of preferably  greater than 99.99%. Ideally, the conveyor surface made from silicon possesses the same level of purity as the silicon fragments which are to be conveyed.
The conveyor surface is moved by means of rapid vibrations of preferably low amplitude, in particular forward-upward and backward-downward. The silicon fragments resting on the conveyor surface are thus subjected to a progressive forward movement which ranges from an abrasive movement to a throwing movement. During this movement, all sides of a silicon fragment face successively toward the hyperpure silicon surface. As a result of the grinding and throwing movement of the silicon fragments on the hyperpure silicon surface, the edges are broken and contamination adhering to the surface is ground off. As a result, uniform rounding of the fragments can be observed. The dust formed is advantageously separated from the flow of silicon fragments, for example by means of an extraction unit. This unit is preferably located along the conveying path, particularly preferably located at the discharge end. In addition, magnetic particles are separated from the flow of silicon fragments by means of an electromagnet or permanent magnet which is arranged, for example, at the discharge end.
In a preferred embodiment of the present invention, the silicon fragments which have been rounded by means of the first vibrating conveyor unit are guided over a second vibrating conveyor unit. The conveyor surface of the second vibrating conveyor unit preferably has passage openings, such as for example gaps or holes. In the process, the silicon fragments are conveyed on the conveyor surface, which may preferably be of planar, trough-like or tubular shape and is made from hyperpure silicon. By means of this second vibrating conveyor unit, the rounded silicon fragments are rounded further. The rounding results in the silicon fragments losing about 0.5% by weight and in a reduction in the metal contamination level from 600 ppb to 100 ppb. The silicon fragments are classified and separated and dust is removed by means of these passage openings.